1. Field of the Invention
The present invention relates generally to a system for detecting and analyzing polarized radiation incident with a skewed polarization plane while utilizing a radiation filter/detector structure having combined wavelength sensitive and polarization sensitive characteristics.
More particularly, the subject invention pertains to a system for detecting polarized radiation incident with a skewed polarization plan while utilizing radiation filter/detector structures fabricated on a single substrate in which different areas of the substrate are coated with different wavelength interference filter coatings, and further wherein polarization filters having both parallel and perpendicular polarizing filtering characteristics are applied onto the substrate in association with the different wavelength interference filter coatings.
As is well known and recognized in the polarization arts, polarization measurements are frequently standardized by taking the measurements with the polarization lines of the polarization filters positioned either parallel or perpendicular to a principal plane which is positioned in a special perpendicular position. As is known in the polarization arts, the principal plane is a plane defined by two intersecting lines, with the first line being defined by the direction of the incident radiation onto the surface from which the linearly polarized radiation is being reflected, and the second line being defined by the direction of observation of the linearly polarized radiation reflected from the surface.
A special case exists when the principal plane also contains a third line extending vertically, normal from the surface being observed and detected. In that special case, both directions (parallel .parallel. and perpendicular .perp.) of the interference filter lines can be oriented to be either parallel to or perpendicular to the principal plane, which enables the linear polarization of the light being detected to be fully analyzed as to its polarization characteristics.
However, in many instances, the principal plane will not contain a third line extending vertically, normal from the surface being observed and detected. In those instances, the principal plane is skewed relative to that third normal line, and can be referred to as a skewed principal plane. With a skewed principal plane, a complete analysis and characterization of the polarization of the radiation requires a second filter/detector, with the direction of its lines of polarization (both parallel .parallel. and perpendicular .perp.) being positioned at an acute angle, preferably 45.degree. for ease of analysis, relative to the lines of polarization (both parallel .parallel. and perpendicular .perp.) of the first filter/detector.
The radiation filter structures as described herein are suitable for use with focal plane array radiation detectors. The radiation filter structures may be used to produce color (wavelength) and polarization sensitivity characteristics for recently developed square micro infrared detectors having sizes between 0.001 and 0.004 inches. These new detectors are composed of materials such as indium antimonide, platinum silicide and mercury-cadmium-telluride, for instance.
There is a need in advanced military infrared systems for wavelength dependent polarization filters which arises because military targets and their backgrounds have different and distinguishable infrared polarization characteristics. Moreover, such military targets and their backgrounds also have polarization properties of both emitted and scattered radiation. Although present military electrooptical systems do not specifically have polarization sensitivity designed into them, recent developments in the mathematical analysis of such optical systems allow polarization sensitive systems to be designed and constructed.
2. Discussion of the Prior Art
Interference optical filters and their manner of construction and characteristics are well known in the optical arts. U.S. Pat. Nos. 3,771,857 and 3,981,568 are of particular interest to the present invention by disclosing technology for producing multifrequency striped interference filters which can be utilized in the present invention. U.S. Pat. No. 3,981,568 in particular discloses a technological method for producing a striped dichroic filter on a substrate. The surface of the substrate supports a first set of spaced parallel stripes capable of transmitting or reflecting a first color and a second set of spaced parallel stripes capable of transmitting or reflecting a second color. The first and second sets of stripes are disposed parallel to each other and in abutting relationship with no space or overlap between adjacent stripes. Third and additional sets of stripes for reflecting other colors may also be provided pursuant to the disclosure of this patent. In the method for making the striped dichroic filter, a dual lift-off procedure is used which utilizes both metal and resist lift-off techniques that are mutually exclusive.
Polarization optical filters and their manner of construction and characteristics are also well known in the optical arts. Polarization can be useful in the detection of specific targets as the polarization properties of particular targets frequently differ from the polarization properties of background radiation. The production of polarized radiation is quite common in nature, and occurs in sky and sunlight reflection, scattering and emission from water and land surfaces, roads and highways, windows, and vehicle bodies, to name a few instances. However, the use of polarization analysis in remote sensing has often been limited to astronomy, where every bit of information available in the optical spectral region must be utilized in analyses.
In order to sense or detect polarization in an electromagnetic wave, an optical element is necessary that responds nonuniformly to the vector directional property of the incident radiation. Such devices are available as polaroids, wire grid polarizers, dichroic polarizers, piles of plates, double refraction prisms, or asymmetrical scatterers. The oldest instrument for analyzing polarization is the wire grid which was first used as a polarizer in 1888 to evaluate the properties of the then recently discovered radio waves. By orienting the grid either parallel or perpendicular to the electric vector of the radiation, the electric field is either short circuited or permitted to pass therethrough, respectively.
This same principle was extended into the infrared in 1911 for polarization analyses of radiation from 24 to 314 .mu.m. One recent application was in 1960 in which wire grids were made by evaporating gold onto a plastic replica of a diffraction grating. Such grids are available commercially from Perkin-Elmer with transmission in the wavelength range from 2.5 .mu.m to well beyond 333 .mu.m. The polarization properties of these wire grid polarizers are shown in FIG. 6., which is a plot of the ratio of the intensity of the transmitted parallel component of polarization to the perpendicular transmitted component as a function of wavelength. The larger the ratio, the better the performance as an analyzer.
In 1938 the chemical analog of the wire grid was invented. Long, thin polymer molecules aligned nearly parallel to each other, with high conductivity produced by free electrons associated with iodine atoms in the molecules, serve to absorb the electric field parallel to the molecules, and the perpendicular component passes through with a small absorption. Pursuant thereto, various sheet polarizers are available from the Polaroid Corporation with usable wavelength ranges covering the region from 375 nm to 2.0 .mu.m. Minneapolis Honeywell also manufactures ultraviolet polarization analysers.
The polarization filtering techniques described hereinabove are applicable and pertinent to the detection system of the present invention to the extent that they can be utilized to produce a filter structure on a substrate as described in greater detail hereinbelow.